1. Field of the Invention
This invention relates to a projecting lens for use in a projection television wherein an image formed on a projection cathode-ray tube (CRT), or the like, is projected onto a large-sized screen. This invention particularly relates to an optical coupling type (OC type) of projecting lens, wherein a liquid is filled between a lens and a CRT in order to keep the contrast ratio high and to keep the CRT cooling efficiency high.
2. Description of the Prior Art
Projection televisions are provided with a projecting lens for projecting an image, which has been formed on a CRT surface, or the like, onto a large-sized screen. Recently, projection televisions are widely used in theaters, exhibition halls, airplanes, and the like.
As the projecting lens for projection televisions, it is desirable that an OC type of projecting lens be used which has a good cooling efficiency and a good contrast ratio.
Projection televisions are also used as terminals for projecting images obtained from VTR's, images obtained from video cameras, images which the televisions have received, computer graphic images, and the like. Recently, the projection televisions are required to have an increasingly higher resolution.
As the OC type of projecting lens which is considered to provide images of a high resolution, a projecting lens disclosed in U.S. Pat. No. 4,900,139 has heretofore been known.
Projection televisions can be classified into a front screen type and a rear screen type.
In the cases of the rear screen type of projection television, the distance between a screen and a projecting lens is set by a rear projection box when the projection television is delivered to users. On the other hand, in the cases of the front screen type of projection television, the distance between a screen and a projecting lens is set by the user. Therefore, the front screen type of projection television should satisfy the requirement that the distance between the screen and the projecting lens can be varied over a wide range (i.e., that the variable power range can be kept wide).
However, with the projecting lens disclosed in U.S. Pat. No. 4,900,139, as shown in Table 1, Table 4, Table 5, and Table 8 in its specification, the magnification range is respectively 16.42, from 10.70 to 12.03, from 10.00 to 10.59, and from 8.13 to 10.87. Specifically, the variable power range is 1.34 times at the most. On the other hand, with an air coupling type of projecting lens, which is illustrated as a comparative example in U.S. Pat. No. 4,900,139, the magnifications range from 10 to 60, and therefore the variable power range is 6 times. The variable power range of the disclosed projecting lens is very much narrower than that of the air coupling type of projecting lens.
As described above, with the aforesaid conventional technique which is known as the OC type of projecting lens capable of forming an image with a high resolution, the variable power range is narrow. The reasons for the above are that, if the variable power range is kept wider, a field flattener lens (i.e., a concave lens which has a large center thickness and is combined integrally with a CRT surface with a liquid sealed therebetween) and the other projecting lens elements cannot be moved together along the optical axis direction. Therefore, the distance between the field flattener lens and the other projecting lens elements changes largely, and the image quality at the peripheral regions and the center region of an image area becomes worse as the magnification goes beyond a reference magnification. Even if measures, such as insertion of a floating lens, are employed, it will be difficult to compensate for the bad image quality.
Accordingly, in cases where the conventional technique is employed as a projecting lens for a projection television, particularly for a front screen type of projection television, the problems occur in that the variable power range cannot be kept wide and it is necessary for several kinds of projecting lenses to be prepared in accordance with, for example, different screen sizes.
Also, with the aforesaid OC type of projecting lens, the temperature of the liquid sealed between the field flattener lens and the CRT surface becomes very high due to heat from the CRT surface, and therefore the lens housing expands. As a result, the position at which the projecting lens is mounted varies, and the projected image becomes unsharp.
In order to compensate for the unsharp state of the projected image, it will be efficient that plastic lenses exhibiting a large variation in the refractive index due to an increase in the temperature are utilized positively. However, the problems have heretofore been encountered in that the conditions, under which the plastic lenses should compensate for the unsharp projected image, and the conditions for widening the variable power range do not match with each other. Therefore, it often occurs that, under certain conditions, an increase in the effects of compensation for the unsharp state of the unsharp projected image results in a narrower variable power range.